The invention relates to environmental control systems. More specifically, the invention relates to a two-spool air cycle machine for an aircraft environmental control system.
Most aircraft air environmental control systems operate on an air cycle refrigeration principle. Compressed air is obtained from a compressor section of the aircraft's main engine, cooled with ambient air to near-ambient temperature in an air-to-air heat exchanger and then expanded in an air cycle machine to provide a stream of cooled, conditioned air. The conditioned air is supplied to the passenger cabin. Although somewhat expanded, the conditioned air also pressurizes the cabin.
The air cycle machine typically includes one or two cooling turbine sections for expanding the air. Cascading two cooling turbine sections generally results in a more efficient thermodynamic cycle and allows the air to be cooled to subfreezing temperatures. Less subfreezing air is used to cool the aircraft cabin. Consequently, the size of the air-to-air heat exchanger can be reduced.
The two turbine sections may be arranged in separate spools. However, packaging the two spools can present problems. Ducting between the two spools, heat exchangers and water separation equipment can become complex.
U.S. Pat. No. 5,309,735 discloses an air cycle machine having two turbine wheels, a compressor wheel and a fan rotor that are secured to a single shaft. Resulting is a "four wheel" air cycle machine. The two turbines may be cascaded.
However, there are also certain problems associated with this four wheel air cycle machines. The fan and compressor are forced to operate at the same speed. However, the fan usually operates more efficiently at lower speeds, whereas the compressor usually operates more efficiently at higher speeds. Thus, mounting all four components on a single shaft sacrifices system performance and adds to the energy input requirements.